Trivalent Metal Lewis Acids Activate CO2 in Transfer Hydrogenations.

Abstract

Using γ-terpinene as a bio-derived H₂ surrogate, trivalent metal MX₃ (M = Al, Ga, In, Yb, X = Cl, OTf) Lewis acids (LAs) catalyze CO₂ hydrogenation to formate, yielding p-cymene as the by-product. The resulting formate produces up to 91% N-formamides in tandem hydrogenation-coupling reactions and up to 95% heterocycles, including azoles, via further in situ transfer formylation to ortho-substituted anilines and cyclization at 130 °C and 4 bar. But In(OTf)₃ and a Lewis base fail to abstract a hydride from γ-terpinene. Unlike other LAs and transfer hydrogenation catalysts that induce hydride abstraction from 1,4-cyclohexadiene(s) over B(C₆F₅)₃, alkali earth or noble metals, MX₃ LAs activate CO₂, so CO₂ can directly accept a hydride from γ-terpinene during formate synthesis, as shown by density functional theory calculations. This triple role of MX₃ LAs in promoting (1) CO₂ activation, (2) tandem coupling reactions, and (3) transfer formylation at low pressure paves the way for sustainable CO₂ hydrogenation processes, leveraging bio-derived H₂ surrogates to develop efficient carbon capture and utilization systems and to synthesize valuable compounds from renewable feedstocks.